In-situ determination of thickness and electrochemical properties of barrier oxide film on impure aluminium in aqueous solution

Lead is present as a trace element (ppm level) in nearly all commercial aluminium alloys. The objective of this work is to investigate the changes incurred by the presence of a small amount (20 ppm) of alloyed Pb and heat treatment on the properties of the barrier oxide in relation to those on pure aluminium, in chloride free acetate buffer. The potential range of interest was - 1.1 to - 0.1 V versus saturated Hg/Hg2SO4. The methods used were electrochemical impedance spectroscopy (EIS) and chronoamperometry. The film growth data obtained by chronoamperometry was analysed by use of Cabrera-Mott inverse square logarithmic law. The activation energy for film growth, obtained from this analysis, decreased with increasing heat treatment time and temperature, along with deleterious changes in the electrochemical properties of the oxide, indicating reduced passivity. The steady state data obtained by EIS showed decreasing oxide resistivity with increasing heat-treatment temperature of the samples in the range 300-600 A degrees C. These changes were attributed to reduced passivity caused by increased segregation of Pb at the aluminium substrate-film interface. [GRAPHICS] .